Private branch exchange automatic telephone system



Dec. 26, 1944. BAKKER 2,365,996

PRIVATE BRANCH EXCHANGE AUTOMATIC TELEPHONE SYSTEM Filed Aug. 12, 1942 14 Sheets-Sheet l PuBL /6 Fax. arr/c5 4 EXCH.

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Dec. 26, 1944. P. BAKKER 2,365,996

PRIVATE BRANCH EXCHA NGE AUTOMATIC TELEPHONE SYSTEM Filed Aug. 12, 1942 14 Sheets-Sheet 2 PB.X. OFF/CE 14.

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Dec. 26,1944. P. BAKKER 2,365,996

PRIVATE BRANCH EXCHANGE AUTOMATIC TELEPHONE SYSTEM Filed Aug. 12, 1942 14 Sheets-Sheet s PBX. OFF/CE B.

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P. BARKER 2,365,996 PRIVATE BRANCH EXCHANGE AUTOMATTLC TELEPHONE SYSTEM Filed Aug. 12, 1942 14 Sheets-Sheet 4 Dec. 26, 1944.

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P. BAKKER 2,365,996

PRIVATE BRANCH EXCHANGE AUTOMATIC TELEPHONE SYSTEM Dec. 26, 1944.

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PRIVATE BRANCH EXCHANGE AUTOMATIC TELEPHONE SYSTEM Filed Aug. 12, 1942 14 Sheets-Sheet 9 EFE P. BAKKER Dec. 26, 1944.

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P. BAKKER Dec. 26, 1944.

PRIVATE BRANCH EXCHANGE AUTOMATIC TELEPHONE SYSTEM Fil ed Aug. 12, 1942 14 Sheets-Sheet 13 aux IUD Iii/517707": Fzer' Bakk 21'' P. BAKKER Dec? 26; 1944.

PRIVATE BRANCH EXCHANGE AUTOMATIC TELEPHONE SYSTEM [filer-175T".- Fzer' Bakk E1" Fig. 1, part 1.

Patented Dec. 26, 1944 PRIVATE BRANCH EXCHANGE AUTOMATIC TELEPHONE SYSTEM Pier Bakker, Chicago, Ill., assignor to Kellogg Switchboard and Supply Company, Chicago, Ill., a corporation of Illinois Application August 12, 1942, Serial No. 454,624

19 Claims.

This invention relates to private branch exchange automatic telephone systems. The principal object of the invention is the provision of new and improved circuit arrangements for use in a private branch exchange to enable the several desired features of service to be given more efliciently and satisfactorily.

An additional object is the provision of suitable circuit arrangements for use in multi-office private branch exchanges.

GENERAL DESCRIPTION It has been chosen to illustrate the invention as applied in meeting the requirements. of a specific installation wherein the branch exchange includes two offices interconnected with each other by trunk lines, and wherein one of the offices is interconnected with the public exchange.

Referring now to the accompanying drawings comprising Figs. 1 to 14,

Fig. 1 is a schematic diagram of the arrangement in the private branch exchange and the trunk-line interconnection with the public exchange;

Figs. 2 to 11 are circuit diagrams; and

Figs. 12 to 14 show the way in which the sheets on which Figs. 1 to 11 are drawn are intended to be arranged.

The system arrangement Referring now to Fig. 1, comprising parts 1 to 3, the private branch exchange (P. B. X) com prises offices A and B, interconnected by four two-way trunk ofiices A and B, interconnected by four two-way trunk lines, of which trunk lines B-Tl and B-T4 are shown. Ofiice A of the P. B. X is interconnected with the public exchange by way of six two-way trunk lines (or P. B. X lines) of which lines A-Tl and A-TG are shown. An attendants cabinet 500 is located in oflice A and is provided with key equipment enabling the operator or attendant thereat to answer and extend calls incoming from the public exchange; answer calls from subscribers of the P. B. X; originate calls to P. B. X lines; and to set up tail-to-tail connections between public-exchange lines and P. B. X lines.

Olfice A The connections in Office A of the P. B. X are set up through finder-connector links shown in In the specific installation depicted, fourteen of these links are provided, of which links Ll, L2, and LM are shown. The

fourteen finders are presented by finders Fl, F2, and FM, while the fourteen connectors are represented by Cl, C2, and CM.

Each of the connectors CI to C l4 has ten levels (groups) of bank contacts (levels 1 to 0), each level serving a separate ten-line group. These levels are served respectively by the group cables G-l to (3-0, each such group cable carrying the tip, ring, and sleeve conductors for a separate ten-line group. The group-one cable G-l serves the ten illustrated line circuits LC-Zll to LC-Zlll and attached P. B. X subscriber lines. Each of the other group cables, G4 to G-ll, similarly serves a separate group often line circuits and attached P. B. X lines.

The cables G-l to (3-0 serving the one hundred P. B. X lines and line circuits are multipled to the corresponding levels 1 to 0 of the finders Fl to FM. The finders have two additional levels served respectively by group cables G-ll and G42. Group cable G-ll contains ten line multiples extending respectively to line circuits LC-l It to LC-l Ill, while the line multiples contained in cable G-IZ extend respectively to line circuits LC-lll to LC-l20.

As many of the line circuits LC-l II to LC-l l0 and LC-l2l to LC-l20 as are needed are used to terminate the incoming branches of the public-exchange trunk lines and the incoming branches of the interoflice P. B. X trunk lines. Specifically the trunk-line branches incoming from oflice B are terminated respectively at line circuits LC-lll to LC-llfl, and the incoming branches of the public-exchange trunk lines are terminated at line circuits LC-l25 to LC-IZD. Additionally, line circuits LC-l2l and LC-l22 terminate call lines ACL and CCL extending thereto from the attendants cabinet 500.

Each group of ten line circuits is provided with a mark cable containing conductors controlled by the line relays to control the starting and setting of the finders Fl to F4. The cables for the three groups of line circuits illustrated in Fig. 1 for oflice A are M-l, M-l l, and M-l2.

The twelve mark cables M-l to M-l2 extend separately to the group relays indicated at GR. These group relays cooperate with the distributor indicated at DR to cause idle ones of the finders Fl to FM to be set successively on the successive calling lines.

In addition to giving access to any one of the full number of one hundred P. B. X subscriber lines, the connectors CI to CM must be arranged to provide additional services. These additional services comprise making connection with the conference line CL when the conference number 41 is dialed; making an outgoing connection over an idle public-exchange trunk line when the publie-exchange call number 51 is dialed; making an outgoing connection with an idle one of the P. B. X interofiice trunk lines when the Offlce-B number 61 is dialed; making calling connection with the code-call equipment CCE when either of the code-call numbers 71 and 81 is dialed; making answering connection with the code-call equipment CCE when the code-answer number 91 is dialed; and making connection with an idle one of the trunk'lines extending to the attendants cabinet 500 when the attendant's number 01 i dialed.

For this purpose, each of the connectors Ci to CI4 is provided with one additional set of contacts for each of the called numbers 41 to 01. These contact sets are indicated below the levels 1 to of the connectors.

The contact sets 4| of the connectors CI to CI4 are connected in multiple on the line side thereof to the conference line CL.

The line side of the contact sets 5| of the connectors CI to C l 4 extend respectively to the selective'switches Al to Al 4, which have access in multiple to the outgoing branches of the main-ex-' change trunk lines A- Tl to A-Tli. The selective switches Al to Al 4 are individual'respectively to the connectors CI to CI4.

Similarly, the line side of contact sets 6| of the connectors CI to CI4 extend respectively to the selective switches Bl to B which have access in multiple to the outgoing branches of the P. B. X interofilce trunk lines B-Tl to B-T4.

The line side of the contact sets ll of the connectors CI to Cl 4 are connected in multiple to call line H extending to the code-call equipment CCE; the line side of the contact sets 8| of the connectors CI to CI4 are connected in multiple to the call line 8| extending to the code-call equipment CCE; and the line side of the contact sets ill of the connectors CI to CI4 are connected in multiple to the answer line 9| extending to the code-call equipment CCE.

The line side of the contact sets CI of the connectors CI to CI4 extend individually to the selective switches SI to S" which have access in common to the thre trunk lines extending by way of the attendant's trunk circuits ATC-l to ATC-3 respectively to answering equipment at the attendant's cabinet 500. I

The equipment at the attendant's cabinet 500 includes keys Kl to K6 associated respectively with the trunk circuits TCI to T06, interposed respectively in the main exchange trunks A-Tl to A-T6. Each of the keys Kl to K6 has a trunk position and a local position as is shown for the key Kl in Fig. 5. Attendants telephone AT is connected to attendant's line 225, multipled into relay contacts in each of the trunk circuits TCI to T06. controlled by relays under the control respectively of the keys Kl to KS, as is shown in Fig. 3 "for the trunk circuit TCI.

Additionally, the attendant's cabinet includes keys K1 and K8 associated with the attendant's 'call line ACL; keys K9 and Kill associated with the conference line CL and the conference call line CCL; keys Kl I to Kl3 associated respectively with the attendant's trunk circuits ATC-l to ATC-3; and a control key K which is used to enable the attendant to make an offering or verifying connection with a busy line.

The repeaters Rl to R4 in ofllce A are i $rl d aseaoee in the outgoing branches of the interomce trunks B-Tl to B-T4 respectively.

The code-call equipment CCE may be of the type shown inv the application of Bert A. Wallace, Serial No. 366,041, filed November 18, 1940, now Patent No. 2,292,183. This equipment has the code signal line CSL extending therefrom, to which any desired number of signal devices SI to SX'may be multiply connected.

The arrangement in Office B of the P. B. X. shown in part 3 of Fig. 1, is similar to that described in connection with Oflice A, shown if. parts 1 and 2 of Fig. 1. The similarity between the parts is made clear by applying the same reference characters to the equipment in Ofllce B as is applied to the corresponding equipment in Omce A, the reference characters applied in Office B being primed.

Oifice B is illustrated as including twelve finderconnector links M to LII, extending respectively from the finders Fl to Fl! to the connectors CI to Cl2.

Certain of the apparatus employed in Office A is not furnished in Office B, being not required. That is, no attendants cabinet and associated equipment are required in Oflice B. nor terminating facilities for public-exchange lines. Accordingly, only five selective sets of contacts (4|, and .H to OI) are required to be added to the connectors CI. to CH, of Oflice B. These five selective contact sets for each of the connectors -in Oflice B are sets 4| for the conference line CL; sets II to ill for the two call lines and one answer line extending to the code-call equipment CCE; and contact sets OI leading individually to the selective switches Bl to Bl2', which have access in common to the outgoing branches in Office B of the interofiice trunk lines B-Tl to B-T4.

The circuit drawings The system having been described generally, a general description will now be given of the detailed circuit drawings comprising Figs, 2 to 11. These circuit drawings all show equipment located in Office A of the P. B. X.

Th main exchange trunk line A-Tl and the associated trunk circuit TCI of Fig. 1 are shown in Figs. 2 and 3;

The'selectiveswitch Al of Fig. 1 is shown in Fig. 4, along with the circuit arrangement of line circuits LC-l2l to I20 of Fig. 1;

The circuit details of the attendants cabinet 500 of Fig. 1 are shown in Fig. 5, which drawing shows also the attendant's trunk circuit A-Tl of Fig.

Fig. 6 shows the circuit arrangement of line circuits LC-2ll to LC-illll serving the first tenline group of the Ofllce-A P. B. X subscriber "lines; I

Fig. 10 shows the circuit arrangement of the distributor DR of Fig. 1; and e Fig. 11. in three parts, shows the connector CI of Fig. 1 together with the link Ll between this connector and the finder Fl of Figs. 1 and 9.

Special features The following are certain special features of the system not particularly covered by the foregoing discussion:

1. As is customary, the subscriber lines of the P. B. X are divided into two classes, non-restricted lines and restricted lines. Calls may be made to the public exchange from the non-restricted lines, but not from the restricted lines. Calls from the public exchange may be made to all lines whether restricted or not, in the discretion of the P. B. X attendant. The attendant may set up a tail-to-tail connection between a line reached by way of the public exchange and any P. B. X line whether restricted or not.

In the illustrated installation, the restrictedservice arrangement is carried out by a separate pair of restricted-service leads .in each of the finder-connector'links arranged to deny the attempted connection (by operating the busy relay of the connector) provided the call is from a re- 'stricted level of the concerned finder and provided the call i attempted to one of the trunk lines leading to the public exchange. The restricted-service leads of the link Ll (Figs. 9 and 11) are the leads RSI and RS2, which may be closed by any desired finder group (level) relay. The connection of conductors RSI and RS2 to the contacts of any level relay of the finder Fl effectively restricts all the lines (subscriber lines or trunk lines) of that level as regards calls to the trunk lines leading to the public exchange.

2. Each of the attendants trunk circuits associated respectively with the trunk lines connected with the public exchange, exemplified by the trunk circuit TCI (Figs. 1 to 3), is so arranged that actuation of the associated key in the attendants cabinet to what is designated as the trunk position sets up a locked, seized condition in the trunk circuit to give answered supervision in the public exchange if the key is thrown in response to a call received in the P. B. X, and to originate a call to the public exchange provided the associated trunk line is not in use when the key is so actuated. The clearing out of the locked condition when the concerned trunk is extended to a local P. B. X line is accomplished automatically responsive to the receiver being replaced on the connected P. B. X line. Otherwise, the attendant clears out the locked condition by depressing the hookswitch of the attendants telephone with the trunk key actuated.

Actuation of the above-named trunk key to local position causes extension of the incoming branch of the associated trunk line to an idle connector and places that connector under control of the attendants calling device. An interlocking circuit arrangement renders the actuation of the above-named key to local position ineffectual until after the attendant has first actuated the key to trunk position to set up the abovementioned holding condition. By this arrangement, the attendant is prevented from taking the local end of the trunk l ne for use without having previously actuated the key to trunk position to cause both ends of the concerned trunk line to be guarded.

3. The attendant is provided with what is termed an offer or busy-verification key, by means of which she can unbalance the current fiow in the tip and ring conductors of any local connection. The connectors are arranged to respond to this unbalanced current flow in the event that the called P. B. Xline is busy to complete a talking and listening circuit from the attendants cabinet to the busy line whereby the operator can verify that the line is in fact busy and can olfer to complete the connection to the busy line.

4. In each oillce of the P. B. X, a non-busy or conferenceline is provided whereby at a prearranged time a number of separate P. B. X subscribers may call the conference line and thereby be placed in common communication to enable a conference to be held. Whenan interofiice conference is required, the attendant, who has access to a multiple of the conference line in the local P. B. X office, may extend a connection from such conference line over a single one of the interoffice P. B. X trunk lines to the conference line in the other ofllce. This arrangement results in a great saving in the number of interofiice trunk lines required. Without it, all persons joining in a conference would have to call the conference number in one or the other of the P. B. X ofiices thereby temporarily withdrawing from service as many interofiice trunk lines as there might be subscribers in one ofiice desiring to make connection with the conference line in the other ofiice.

5. Arrangements are provided for causing a continuous rotation of the use of the links and trunk lines on successive calls. This feature is particularly valuable durin periods of light traffic. The rotation of the use of the finder-connector links is secured by the progressive action to be hereinafter described of the distributor DR of Figs. 1 and 10. The distributor is so arranged that it takes the next succeeding idle finder for use on the next call even though the next call is not made until the call last made has been cleared out and the last used finder has again become idle.

The selective switches through which access is given from the connectors to the trunk lines are provided with a slip arrangement whereby successive ones of these switches have a different choice access to the trunk lines. This arrangement cooperates with the progressive action of the distributor to cause rotation of use of the trunk lines.

6. Specific features of the distributor arrangement as exemplified in Fig. 10 for the distributor DR are that the distributor searches for an idleindicating battery potential on the test leads of the finders; it contains a pair of interlocked re lays which positively cause a progression of the distributor of at least one step on each call handled thereby; and it contains a clear-out timer wh ch operates on any call to clear out the oper ated group relays in the event that they do not clear out as the result of the attempted or comple e finder ope ation.

'7. A specific feature of the selective switches such as Al to A14 and SI to SM of Fig. 1, exemplified at Al in Fig. 4, is that each constitutes a hunting switch which makes connection successively with the trunk lines accessible thereby until an idle one is found, whose idle condition is indicated by the presence of battery potential on the test conductor thereof.

8. A specific feature of the switches such as BI to BM of Fig. 1, exemplified by the circuit drawing (Fig. 8), is that each of these switches comprises a group of relays corresponding respectively to the trunks accessible to such switch, and a common allotter is provided comprising allotter relays common to all the switches and corresponding respectively to the trunks accessible to such switches. The allotter arrangement is such that upon seizure of any of the switches, the first idle trunk accessible thereto is taken for use immediately by the energization of the corresponding relay of the seized switch. The common allotter carries an individual chain circuit for each of the switches, and the point of access to the chain circuit is rotated on successive switches whereby rotation of the use of the trunk lines on successive calls is eflected with a straight, nonslipped, multiple of the trunk lines to the several switches of the group.

9. Divided-ringing, two-party service is available on each P. B. x subscriber line by assigning two hundreds digits (2 and 3) to the same group of one hundred P. B. X subscriber lines. When the normal hundreds digit (2) is dialed (as in #210), connection is madenormally to the called line, ringing current being projected over the tip conductor. But, when the alternate hundreds digit (3) is dialed (as in #310) the connector is altered thereby to project ringing current over DETAILED Drzscnrrrron A. A local call; Ofllce A aseaoae I034, chain contacts controlled by the first lower armatures oi relays GC-I to GC-I 2, chain-in con- A description will first be given of the opera.-

purpose, it will be assumed that the subscriber at substation #211 (Figs. 1 and 6) desires to converse with the subscriber at substation #210 on the two-party line #2l0;310 (Figs. 1 and 6).

Referring first to Fig. l, the desired connection, when completed, is from the calling station #211 by way of the correspondingly numbered line multiple in the group-one cable G-I; through first-level contacts of one of the finders FI to F; through the corresponding one of the links LI to LI; and through the first level contacts of the concerned one of the connectors CI to C, to the called line #2103310, by way of the correspondingly numbered line multiple in group-one cable G-I Al. Marking the calling line calling condition and marking the identity of the calling line in the group relays GR (Fig. 9).

Ala. Units marking Assuming that the group relays GR of Fig. 9, and the distributor DR of Fig. 10, are in normal condition at this time, subgroup relay GA-I and tens-group relay GC-I operate in series over the following circuit path: from ground on conductor 1-5 in cable M-I, through the winding of relay GA-I, lower contacts of relay GB-I, the winding of relay GC-I, chain-end conductor I033, lower contacts of start relay I005, chain-out conductor ductor I032; contacts of clearout-timer relay,

I002, to battery through the associated resistor.

Upon operating over the above-traced circuit, subgroup relay GA-I connects the associated conductors 1 to 5 in cable M-I respectively to the units conductors 1 to 5 in units cable 800-U, thereby extending ground potential from the grounded conductor 1 in cable M-I to conductor 1 in units cable 900-U.

Relay GA-I, at its lower contact pair, disconnects relay GB-I to prevent operation of the lat ter. relay in the event that a call should be made concurrently from one of the lines 6 to 0 in the same ten-line group.

Alb. Tens marking Upon operating over the above-traced circuit,

- relay GC-I locks itself to the chain circuit at its A2. Starting the distributor DR At its upper contacts, relay GC-I applies ground potential to start conductor I03I, thereby, closing a circuit, through the front contact of the normally operated busy relay IO0I, for start relay I005 of the distributor DR. Slow-acting start relay I005 operates after a slight interval. Clearout-timer relays I003 and I004 operate responsive to the closure of the front contact of the second upper armature of relay I005. Relay I004 opens a point in the restoring circuit of relay I003, and

' relay I003 disconnects the final clear-out relay I002. Relay I002 is preferably marginally adjusted to prevent its operation in shunt of the resistors associated with the relays I003 and I000 in the event that relay I003 should be slow in operating.

A3. Freezing progress-control relay 1009 At the front contact of its lower armature, start relay I005 applies ground potential to the lower armature of progress-control relay I009. By this operation, the present condition of relay I003 (whether operated or restored) is maintained. If relay I009 is restored at the time, as it is on about half of the calls, ground potential is extended through the back contact of its lower armature to the left winding terminal thereof, thereby preventing this relay from responding to an operation of stepping relay IOI0. On the other hand, if relay I009 is in operated condition at the time, as it is on about half of the calls, the grounding of the lower armature thereof extends ground potential through the front contact of such armature to the right-hand winding terminal of the relay to close a locking circuit for it. The utility of this arrangement will appear later.

A4. Distributor operation On the first call to the distributor DR after it has been placed in operation and the battery supply connections have been closed, none of the relays thereof except busy relay IO0I is in operated condition when start conductor I032 is grounded as above described to bring about the operation of start relay I005. Under this condition, the closing of the front contact of the first upper armature of start relay I005 closes an operating circuit for the #1 distributing relay as follows: from ground, through the normally closed contacts controlled by the first upper armature of the #13 distributor relay I023, corresponding contacts of the preceding distributor relays including IOI4 to IOI2, back contacts of the first upper armatures of relays I MO, I 001, and I006, the front contact of start relay I005, to battery through the winding of relay IOII.

Upon operating, relay IOII closes a locking circuit for itself at its first upper armature independent of the contacts of relays I005 to I001 and IMO; at the front contact of its second upper armature, it connects the even operating conductor I041 to the #2 distributor relay IOI2; at its third and fourth upper armatures, it connects the test and connect conductors I034 and I033 of the finder FI to the common conductors I049 and I048; and at the front contact of its lower armature, it closes an operating circuit for stepping relay I 0I0. Relay IOI0 disconnects operating conductor I045 from odd conductor I046 and connects it to even conductor I041 preparatory to operating the #2 distributor relay I 0I2.

A4a. Operating progress relay 1008 The first step has now been taken by the distributor, and progress relay I008 is now to be operated to denote that fact.

At this time, with the upper armature of stepping relay IOI0 in operated condition, and the upper armature of progress-control relay I009 in restored condition, the following circuit is effective for the upper winding of progressrelay I008, in series with relay I 0I2: from ground, through the front contact of the upper armature of start relay I005, conductor I043, contacts of relays I006 and I001, conductor I044, high-resistance upper winding of progress relay I008, normally closed contacts controlled by the first upper armature of relay I008, conductor I045, front contact of the second upper armature of relay IOI0, even conductor I041, front contact of the second upper armature of relay I0. and the winding of relay I0 I 2, to battery.

Relay I008 operates over the above-traced circuit, but relay IOI2 is not operated thereover because of the high resistance of the upper winding of relay I008. Upon operating, progress relay I008 locks its lower winding energized at its lower contacts, through contacts controlled by the lower armature of relay I006, to ground on conductor I04I. At its upper contacts, progress relay I008 connects test relay I 001 to the common test conductor I049 to enable a test to be made over conductor I034 of the busy or idle condition of the first finder FI (Fig. 9). Additionally, relay I008, at the front contact of its second upper armature connects conductor I044 directly to conductor I045 thereby by-passing the high-resistance upper winding of relay I008 and the upper contacts of relays I009 and IMO. The upper winding of relay I008 is open-circuited at this time by the contacts controlled by the second upper armature thereof. The ground potential on conductor I044 is now extended by way of conductors I045 and I041 to the winding of the #2 distributor relay IOI2, to enable such relay to operate, subject to the control of test relay I001.

A411. Selecting an idle finder further operation depends on whether the find-- er FI (and associated link LI and connector CI) is busy or is idle. Assuming that the finder PI is idle, this condition is marked by batte y potential on conductor I034 received from conduc-j tor H of link LI (Figs. 9 and 11), impressed thereon through the back contact of the lower armature of release relay H04 and the associated resistor. Test relay I001 accordingly operates quickly from this battery potential responsive to being connected up at the upper contacts of progress relay I008. The relative adjustment of th relays is such that test relay I001 operates more quickly than any of the distributor relays IOII to I024 can operate. Accordingly, relay I001 disconnects the grounded conductor I043 from conductor I044 to thereby open the circuit of relay IOI2 before the latter relay can 0D- erate.

At its front contact, test relay I001 closes an operating circuit for switching relay I006. Relay I006 thereupon operates and locks itself directly to conductor I043, at the same time disconnectin the armature of test relay I001. At its lower armature, relay I006 connects conductor I042 to the grounded conductor I04I to start the timing operation of the clear-out timer comprising relays I002 to I004, at the same time opening the locking circuit of the lower winding of progress relay I008. Relay I008 thereupon restores. At its upper contacts, it disconnects test relay I001 whereupon the latter relay restores, leaving switching relay I006 operated in its self-locking circuit.

A5. Operating the selected finder At its upper contacts, switching relay I006 applies ground potential to conductor I048 thereby operating connecting relay I of the finder FI through the upper contacts of the operated distributor relay I0 and over conductor I033. Connecting relay 90I thereupon connects the conductors 1 to 12 in the tens cable 900-T respectively to the finder relays TI-F to TI2-F, at the same time connecting the conductors 1 to 6 in the units cable 900-U respectively to the units relays UI-F to UB-F.

Tens conductor 1 in cable 900-T having been grounded at the lower contacts of the operated tens group relay GC-I, level-I relay TI-F of the finder FI now operates. Upon so doing, it connects the thirty conductors comprising the ten line multiples carried in cable G-I respectively to the ten local branches controlled by tne units relays. At its lower contacts, relay TI-F prepares a locking circuit for itself to conductor H of the link LI.

Mark conducto 1 in cable M-I having been grounded at the calling line circuit and having been connected, through contacts of the operated relay GA-I to units conductor 1 in cable 900-U, units relay UI-F of the finder FI operates over the latter conductor. At the back contact of its first upper armature, units relay UI-F opens the battery supply connection to units relay UZ-F to U5-F to prevent a double connection by the finder F in tr e event that two lines in the same subgroup of five are calling contemporaneously. Units 6 switching relay UB-F is not operated at this time because it can operate only when one or another of th relays GB-I to GB-IZ is in operated condition, indicating a call from a line in the second subgroup (6 to of the calling ten-line group.

The tip, ring, and sleeve conductors of the line multiple LM-ZH associated with the calling line #211 are now extended through the upper group A6. Preparing the connector 01 for operation In the connector CI of Fig. 11, thewindings of line relay H03 are connected, through the windings of differential detector relay H03 and contacts of supervisory relay HOI, to the tip and ring conductors of the link LI. The lower windingof line relay H03 is accordingly now energized over the calling line in parallel with the winding of line relay 103 of the calling line circuit LC2I I, the upper winding of line relay H03 being temporarily shunted by the ground connection to the tip conductor of the calling line at contacts of cutofi relay 144.

Upon operating, line relay H03 operates the slow-restoring release relay H04. Relay H04, applies ground potential at the front contact of its lower armature to the hold conductor H of th link LI, thereby locking the operated finder relays UI-F and TI-F. At the front contact of its first upper armature, relay H04 prepares an operating circuit over conductor H31 for the counting relays of Fig. 11, part 3; at the back contact of its first upper armature, it removes ground potential at one point from the common busy conductor I035, leaving this conductor grounded at the similar contacts of each idle connector; and at the front ntact of its first upper armature, it applies ound potential to the joined conductors RSI and H30, whereupon dial-tone relay H20 operates through contacts of hundreds-transfer relay I I I9 to impress a dialtone signal on the tip conductor of the calling line, by way of condenser H34 and contacts of relays HH, Hi3, and Hi0.

At its upper contacts, release relay H04 applies ground potential to sleeve conductor S of the link LI, thereby operating cutoif relay 104 of the calling line circuit LC-IH (Fig. '1). Cutofi relay 104 clears the calling line, whereupon line relay 103 restores. When the calling line is cleared, the calling subscriber hears the dial-tone signal applied atthe upper contacts of dial-tone relay I I20, and is thereby informed that he may begin to dial th digits in the desired number.

A7. Clearing out the group-control relays and distributor The restoration of line relay 103 removes ground potential from conductor 1 of cable M-I thereby opening the initial circuit of units relay UI-F, leaving this relay operated in its locking circuit over conductor H of the link LI. Additionally, if no other line in the first subgroup (lines 1 to of the same tens group is calling,

- the restoration of line relay 103 results in the removal of ground potential of the subgroup mark conductor 1-5, whereupon the operated subgroup and group relays GA-l and GC-I restore. The restoration of relay GC-I results in the removal minate the timing action to be hereinafter de- I scribed of the clear-out timer, as it is necessary iorthis timer to complete its operation only when the normal clearing-out operation now being described does not occur. Any operated ones of the relays I 002 to I004 thereupon restore. Start relay I005 also disconnects ground potential from conductor I043, thereby unlocking and restoring switching relay I006. Ground potential is accordingly removed from conductor I043 and consequently from conductor I033, whereupon connecting relay l restores to disassociate the conductors in cables 900-T and 900-U from the finder FI At the front contact of the first lower armature of start relay I005, ground potential is removed from the lower armature of progresscontrol relay I009, thereby removing the short circuit from the winding of this relay and permitting rela I009 to operate from the ground potential supplied thereto at the front contact of the lower armature of stepping relay IOI0 (held operated by the locked #1 distributor relay IOH). Upon operating, relay I009 prepares a locking circuit for itself at its lower armature, while, at its upper armature, it cooperates with the upper armature of the operated'stepping relay IOI0 to restore the normal short circuit or bypass around the high-resistance upper winding of progress relay I008.

At its lower contacts, start relay I005 rejoins chain-end conductor I033 to chain-out conductor I034 thereby again conditioning the group relays GR. for further operation under the control of any further calling ones of the line circuits.

A8. Dialing the hundreds digit (2) When the calling subscriber at substation #211 (Fig. '1) dials the hundreds digit 2 in the desired number, the usual calling device (not shown) produces twomomentary interruptions in the direct-current bridge across the calling line, thereby causing two momentary restorations of line relay H03, of insuflicient length to permit the slow-restoring release relay I I04 .to follow.

Upon each momentary restoration of line relay H03, an impulse is transmitted at the back contact of the second armature thereof, through contacts of relays III04, IHI, and Hi2, to operating conductor H31. Series relay Hl5 responds to the first of these impulses. Being slowrestoring, it remains operated until the series of impulses is ended. At the left front contact of its first upper armature, series relay HI5 operates the slow-restoring auxiliary relay H", from ground on conductor H30, grounded at contacts of release relay H04.

The first'momentary impulse delivered to operating conductor H31 passes, through series contacts of relays H2I to H21, and through series contacts of driver relays C, B, and A, to counting relay 1. Relay 1 thereupon operates 

